1. Field of the Invention
The invention is based on a semiconductor lasers, and in particular, to a semiconductor laser having a branched cavity.
2. Background Information
Such a semiconductor laser is disclosed in Electronics Letters, Feb. 15, 1990, Vol. 26, No. 4, pages 243-244. It is monolithically integrated on an n-doped indium phosphide substrate. The underside of the substrate is called the base surface. Above a plane that is coplanar with the base surface, there extends a cavity of indium gallium arsenide phosphide (InGaAsP). It is branched and, when seen from the top, has the shape of a "Y". The top view surface of the cavity may also have a different shape, for example the shape of a cross. Significant is that the cavity is contiguous. This can be described in the sense of a topological definition in that the top view surface of the cavity can be considered to be a "singly contiguous region" since it is not composed of several pieces, but of a single piece having a single edge. The cavity lies on the planar surface of an n-doped buffer layer of indium phosphide that extends above the indium phosphide substrate.
Further layers are provided above the cavity. Above the buffer layer, these layers and the cavity form a mesa which has been produced by etching. In the plane that is coplanar with the base surface, the mesa as well as the cavity have a Y shape.
Such a laser with a branched cavity is provided according to the above-mentioned publication as an electrically controllable light source for optical communications transmission systems. In particular, if its metal layer extending above the cavity is subdivided into several electrodes so that cavity regions are created which can be controlled by means of different operating currents, such a laser is distinguished by its emission wavelength being tunable over a very wide wavelength range. This characteristic is of major importance for the intended use as an electrically controllable light source.